1. Field of the Invention
The invention relates to an automatic transmission for a vehicle. More specifically, it relates to the arrangement of the each transmission structural element in the gear train.
2. Description of the Related Art
An automatic transmission for a vehicle is required to multiply the gear ratios for an improvement of driving performance and fuel consumption. According to the requirement, the transmission mechanism tends to transit from the mechanism having four forward speeds to the mechanism having five forward speeds. For achieving further multiplication of the gear stages in the limited mounting space of a vehicle, a reduction in the number of elements of the gear train and a simplification of the mechanism are needed. A gear train, which achieves six forward speeds and one reverse speed with a planetary gear set having minimum shift elements, is suggested in U.S. Pat. No. 5,106,352. In this gear train, six forward speeds are established by transmitting rotation inputted to the shift mechanism and two decelerated rotations decelerated from the input rotation are inputted to the planetary gear set having four shift elements.
In the aforementioned gear train, a number of shift elements for establishing gear stages and a number of needed clutches and brakes are reasonable. However, the gear train has problems which have to be practically improved. In the gear train, two clutches are needed for inputting the decelerated rotations from the reduction planetary gear to two different elements of the planetary gear set. These clutches need larger torque capacities than a clutch inputted a non-decelerated rotation. In the aforementioned structure, the two clutches for transmitting the decelerated rotations are arranged on one side of the reduction planetary gear, and a connecting member for connecting one clutch to one element of the planetary gear set is arranged in a position radially outward of the other clutch. Therefore, the diameter of the other clutch is limited, and it is difficult to obtain the capacity needed by increasing the diameter of the clutch. As a result, an increase in the number of the frictional members is required for obtaining the needed capacity which is usually set based on the diameter of the frictional member and the number of the frictional members. Therefore, the size and the weight of the transmission are increased because of the increase in the axial size.